The present invention relates to a method for providing a pattern-wise photoresist layer on a substrate plate and a surface-protected substrate plate therefor. More particularly, the invention relates to a method for forming a pattern-wise photoresist layer having excellent resolving power and image reproducibility on a surface-protected substrate plate in the so-called dry-film process without removing the surface protecting film from the substrate surface.
As is well known, photolithographic techniques are widely used in a variety of fields including the electronics industry such as the manufacture of printed circuit boards and other precision devices and elements by utilizing various types of photosensitive materials, among which the most conventional ones are those supplied in a liquid form such as a solution of a photosensitive resin for coating or those having an ink-like consistency suitable for silk-screen printing.
When a photosensitive layer is to be formed on the surface of a substrate plate, the photosensitive material in a liquid form is applied to the substrate surface by use of, for example, a roll coater, doctor knife and the like to give a desired thickness of the photosensitive layer. A problem in this case is that the photosensitive material in the liquid form usually has a consistency or solid content not so high as to give the desired thickness by a single coating after drying. The thickness of the dried photosensitive layer obtained by a single coating does not exceed usually 2 to 3.mu.m or, even in the case of a combination of the optimum conditions, 10 .mu.m at the highest. When a large thickness of 30 to 50 .mu.m is desired of the dried photosensitive layer on the substrate surface, therefore, the steps of coating and drying of a liquid photosensitive material must be repeated many times until the desired thickness of the dried photosensitive layer is obtained with large consumption of time and labor.
With an object to avoid the above mentioned practical difficulty in obtaining a relatively large thickness of the photosensitive layer on a substrate surface, the so-called dry-film process has been recently developed in which a lamination called a dry film is prepared in advance by providing a flexible base plate or sheet with a layer of a photosensitive material of a thickness as large as desired and a substrate having a smoothed surface is overlaid with the dry film under pressure through a hot roller with the photosensitive layer in direct contact to the substrate surface so that the photosensitive layer is adhesively bonded to the substrate surface to be ready for pattern formation. Several methods of the dry-film process have been proposed hitherto including a method in which a dry film is directly laminated with the substrate plate followed by image-forming exposure to light and development (see, for example, Japanese Patent Publication No. 45-25231) and a method in which the dry film is provided with a layer of an adhesive by means of which the dry film is adhesively bonded to the substrate surface (see, for example, Japanese Patent Kokai No. 52- 154363).
Turning now to the problem encountered in the storage and handling of photolithographic substrate plates, for example, for the manufacture of printed circuit boards, the substrate surface is not rarely subject to contamination and mechanical damage or surface oxidation to adversely affect the adhesiveness with the dry film and the performance in etching. Therefore, it is usual that substrate plates before use are provided with a surface-protecting film which prevents contamination, mechanical damage and oxidation of the substrate surface. The formulation or composition of the protecting film of course involves a difficult technical problem to be solved because such a protecting film must have adequate adhesion to the substrate surface and satisfy other requirements. For example, poor adhesion of the protecting film to the substrate surface results in falling off of the film during storage or transportation with consequent loss of the protection while a protecting film of too strong adhesion must be removed with great difficulties prior to the use of the substrate plate. Alternatively, a substrate surface may be protected by coating with a grease or the like protecting agent in place of the protecting film. The protection provided by such a protecting agent is, however, not quite effective to protect the substrate surface from mechanical damages and, moreover, the greasy protecting layer must be completely removed by washing prior to use of the substrate plate to cause great troublesomeness.
It has been eagerly desired therefore to develop a novel dry-film process by use of a surface-protected substrate plate free from the above described problems and disadvantages in the prior art method and the inventors have continued extensive investigations therefor arriving at the establishment of the present invention described below in detail.